The present invention is directed to bicycles and, more particularly, to a shift control device for a bicycle transmission.
Shift control devices are used to shift bicycle transmissions from one gear ratio to another. One type of shift control device shown in U.S. Pat. No. 5,400,675, for example, comprises first and second levers that operate a wire spooling device, wherein the first lever is moved from a rest position to an operating position to wind a transmission control wire around the wire spooling device, and the second lever is moved from a rest position to an operating position to unwind the transmission control wire from the wire spooling device. The wire spooling device comprises a cylindrical wire spooling member having a wire winding groove for winding and unwinding the transmission control wire, a plurality of drive teeth circumferentially disposed around the outer peripheral surface of the wire spooling member, and a plurality of positioning teeth also circumferentially disposed around the outer peripheral surface of the wire spooling member. The wire spooling member is biased in a wire unwinding direction by a spring. A drive pawl is mounted to the first lever for pressing against one of the plurality of drive teeth in response to rotation of the first lever from the rest position to the operating position to thereby rotate the wire spooling member in a wire winding direction, and a positioning pawl selectively engages one of the positioning teeth to hold the wire spooling member in a desired rotational position. The second lever is operated from the rest position to the operating position to selectively engage and disengage the positioning pawl to allow the spooling member to rotate in the wire unwinding direction in accordance with the biasing force of the spring.
Another type of shift control device shown in U.S. Pat. No. 5,094,120, for example, also comprises first and second levers that operate a wire spooling device, wherein the wire spooling device is biased in a wire unwinding direction. In this shift control device, the wire spooling device comprises a wire winding plate that rotates integrally with the first lever. An indexing plate also rotates integrally with the first lever, wherein the indexing plate has an indexing slot defining a plurality of indexing teeth staggered on opposite sides of the indexing slot. The second lever has an indexing pin that engages successive ones of the indexing teeth within the indexing slot such that the wire winding plate, indexing plate and first lever together may be held in multiple rotational positions corresponding to the indexing teeth. Operation of the second lever from a rest position to an operating position and back to the rest position allows the indexing pin to move sequentially from one indexing tooth to another. This, in turn, correspondingly moves the wire winding plate, indexing plate and first lever to a plurality of positions in the wire unwinding direction.